ROME—Italy's center-left Democratic Party, led by young Prime Minister Matteo Renzi, won a resounding victory in Sunday's European Parliament elections, garnering more than 40% of the vote and clearly beating the rival euroskeptic party of comedian Beppe Grillo, according to official data.

Based on final Interior Ministry data, the Democratic Party won around 41% of the vote, marking a major success for Mr. Renzi in his first his...